A manufacturing technology pioneered by the applicant is known in the prior art by the term “print-forming.” Examples of this prior art technology are described in U.S. Pat. Nos. 5,348,693 and 7,255,551, incorporated herein by reference in their entirety. The print-forming method is derived from printing technology and uses proprietary “inks” to build multi-material complex parts. One company which has perfected such manufacturing technology is Eoplex Technologies, Inc. of Redwood City, Calif., which describes its manufacturing technology under the trademark “HVPF.” Parts are designed in layers like rapid prototyping, but HVPF™ is a true manufacturing process that allows metals, ceramics, polymers and void spaces to all be designed into the part and thousands of parts to be made at the same time. Parts can contain hundreds of layers with thickness that ranges from microns to millimeters. The number of layers utilized with print-forming is determined by the resolution required by the part. For example some parts made by Eoplex have required over 300 layers and 5 different materials to achieve the proper resolution and function.
Structures that contain different materials can be manufactured in a variety of ways, with various advantages and disadvantages to the manufacturing methods. The simplest method would be to mix the materials into a homogeneous mixture and then form a part using casting, pressing or other forming methods. Concrete is a very basic example of this approach. The mixture in concrete typically consists of Portland cement, sand, gravel and water. The finished parts will be a conglomerate of the sand and gravel held together by the hydrated cement. Such a random amalgamation will typically have physical characteristics that are uniform throughout. However, such homogeneous structures are suitable only for the most basic technology and cannot produce the sophisticated results required in many applications. For example, in the semiconductor industry the manufacture of microchips requires precise placement of silicon, oxides, and doping materials to achieve the required design. This same type of requirement is also needed where semiconductor process cannot be used such as parts that contain different materials including conductors, ceramics, glasses, polymers and dielectrics and where complexity is high. Many processes address some of the requirements in this area but none are able to cover all the requirements and some devices remain either impossible to mass produce, cost prohibitive or both. Examples of the types of processes used include; MEMs, micro-assembly, LIGA, injection molding, embossing, micro-machining and others. Some of these methods work well with one material and a few methods can be modified, at high cost, to handle two materials. However, there are very few technologies that can handle several different materials simultaneously, with complex geometries, in small parts.
A second way to manufacture structures with multiple materials is to place the different materials into separate locations within the structure. For example fiber reinforced structures like graphite golf clubs and fiberglass fishing rods are actually layers of fiber bonded in a polymer matrix. In these composites the key to success is the strength of the bonding between the fibers and the polymers. Failures in these products are typically seen as delamination at this critical juncture such a union of dissimilar materials into separate micro-regions of a structure can be desirable in many applications however, in other applications the need is for a more integrated design of the dissimilar materials and a stronger bond between them.
At least one example where the interface and bond strength between dissimilar materials is critical is when the structure is to undergo repeated stresses which are handled differently by the dissimilar materials. For instance, and as depicted in FIGS. 1, 3 and 4, the prior art structure P might be provided with a first material having a low coefficient of thermal expansion A confined to a portion of the structure P above a boundary B, with a high coefficient of thermal expansion material D confined to a portion of the structure P below the boundary B. When heat H is applied to the structure P (FIG. 3) the material A responds differently than the material D.
In particular, and as depicted in FIG. 4, the material A does not expand as much as the material D. While the materials A, D are joined together at the boundary B, and thus have a tendency to restrain expansion under the thermal stress associated with application of the heat H, these stresses can exceed limits of the materials, resulting in undesirable warping and/or the formation of cracks C. In severe cases the materials can even split apart at the junction. Note from FIGS. 3 and 4 that the ends E remain adjacent each other before the heat H is applied. After the heat H is applied, the ends E have spread differing distances resulting from the different coefficients of thermal expansion, thus causing both the forces tending to warp the structure P and formation of cracks C, particularly along the boundary B.
Concentration of thermal stresses along a boundary is only one problem associated with bonding and joining and building structures with dissimilar materials. As another example, differing materials usually have a significantly different modulus of elasticity so that they exhibit a greater or lesser degree of flexibility when placed under tension, compression or torsion loads, resulting in a similar concentration of stresses along the boundary as that depicted in FIGS. 3 and 4. As other examples, the dissimilar materials might have differing transition temperatures at which strength of the materials is diminished, such that the final structure exhibits non-uniform strength characteristics and associated stress concentrations, when transition temperatures have been exceeded for one material and not the other. Other dissimilar stress characteristics between materials could also be experienced which tend to concentrate stresses along a boundary and otherwise cause the material to delaminate along the boundary or crack along the boundary or otherwise exhibit degraded performance along the boundary in an undesirable fashion.
Furthermore, the materials might be of a type which do not readily bond together or for which there is not a suitable intermediate phase to create bonding. A strong bond at the interface layer is critical since this is the place where stress is concentrated under load and where failure will first occur. If the materials cannot be bonded properly they may not be able to be used together.
Accordingly, a need exists for a technique for forming continuous structures from dissimilar materials in a manner which avoids concentration of stresses, and resulting cracking, warping or other undesirable performance along the boundaries between dissimilar materials.